In a combine harvester, after a crop has been cut, it is passed through a thresher, which acts to separate the grain from the stalks or the straw. The straw is transported to the back of the harvester while the grain and other crop particles drop onto a sieve. The material other than grain (MOG) comprises chaff, short straw and other particles that the thresher has separated from the longer straw stems.
The sieve is reciprocated while air is blown upwards through it. The shaking of the sieve distributes the grain evenly over the area of the sieve and conveys the grain and MOG towards the back of the harvester. The grain that drops through the sieve is collected in a sieve box, from which it is transported to a grain tank. Commonly, a second sieve is reciprocated below the first sieve for further cleaning of the grain sample.
A problem is experienced with combine harvesters when operating on the side of a hill because the grain and MOG tend to accumulate on the downhill side of the sieve. As a result, the layer of crop material on the sieve shows some holes or lightly loaded areas, through which a substantial amount of air can escape. The consequent pressure drop also affects the crop-loaded areas, such that less MOG is lifted up and blown out of the combine harvester. Moreover, the available area of the sieve is not used effectively; grain mingled with MOG remains concentrated near the lower side of the sieve and grain loss occurs because many kernels are conveyed out of the combine harvester, together with the MOG.
This problem is addressed in GB-A-2072050, which proposes altering the angle of oscillation of the entire sieve box. Instead of the sieves being reciprocated parallel to the direction of travel of the combine harvester, they are oscillated at an angle to the direction of travel of the combine harvester. Consequently, the grain tends to be conveyed uphill by the oscillation of the sieves, thereby counteracting the effect of gravity and distributing the grain and MOG more evenly over the surface of the sieves.
In GB 2072050, the grain collecting box, also called cleaning shoe, which holds the sieves, is fitted with guide rollers that are slidably received in cam tracks, which can be pivoted on the frame of the combine harvester to alter the direction of reciprocation of the sieves. Such an arrangement for guiding the movement of the sieves is not however satisfactory because of excessive wear on the cam tracks and because of the limited life of the rolling element bearings used in the guide rollers.
GB 2146218, which is believed to represent the closest prior art to the present invention, provides a drive linkage for reciprocating a sieve that is suspended within a frame of a combine harvester for movement in mutually orthogonal directions. The drive linkage comprises a cranking mechanism for imparting a reciprocating motion to the sieve in the direction of travel of the harvester. The drive linkage further provides a pivot arm pivotably connected at one end to one side of the sieve to cause the sieve to move in an arc centered on a pivot point at the opposite end of the pivot arm as the sieve is reciprocated.
GB-A-2146218 improves on the proposal in GB 2072050 in two ways. First, instead of the grain collecting box being reciprocated, only the upper sieve is reciprocated, thereby reducing the oscillating mass. Second, the guide rollers and cam tracks are replaced by a pivot arm, which constrains the sieve to follow an arc. By altering the position of the axis about which the pivot arm rotates, it is possible to adapt the movement of the sieve to suit different ground inclinations. In this case, the connections at the opposite ends of the pivot arm can be formed as rubber bushes that can operate reliably over prolonged periods.
A disadvantage of the geometry of the drive linkage taught in GB 2146218 is that the sieve no longer moves in a straight line but in an arc. When the sieve is constrained by a pivot arm to oscillate along an arcuate path, its operation is asymmetrical and it is not capable of compensating equally for slopes in opposite directions. This deviation becomes particularly evident when compensating for steeper slopes. Furthermore, the compensation system is controlled by a pendulum system, which allows no adjustment of the trajectory to changing crop conditions, such as crop type, humidity, grain to MOG ratio, etc.